We talked about this a couple of times already. I think it's a good idea
to uniformize the style like this.
To keep this diff readable, I disabled string normalization (which turns
single-quotes into double-quotes). We should probably enable it in the end,
but I don't have a strong opinion on it.
Black's reasoning is that English text is more likely to use apostrophes
than quotes (which I think is true for our code as well).
What do you think?